Outdoor noise monitoring systems are used for long-term measurements of noise, e.g. at roads, airports etc. It is essential to continuously check the accuracy of operation of these systems in order to ascertain that the measurement results are correctly generated. It is preferred if the checking can be performed remotely, without a need for an operator to perform manual procedures at the microphone system. One of the checking procedures is to check whether the system operates according to the initial calibration.
So far, typical outdoor noise monitoring stations used condenser microphones. A condenser microphone can be calibrated by means of an electrostatic actuator that comprises an electrode that permits the application of an electrostatic force to the metallic or metalized diaphragm of the microphone in order to perform the calibration. Alternatively, the equivalent capacitance of the microphone can be measured.
It is also possible to perform acoustic calibration by performing comparison of sound levels received by the measured microphone and a reference microphone. However, as the condenser microphones are relatively large and require high performance preamplifiers, the reference microphone would occupy too much space in the measurement system housing, which must meet strict acoustic requirements. Additionally, such system requires generation of a reference acoustic signal, which makes it hardly applicable in practice.
A US patent application US20140369511 discloses a self calibrating dipole microphone formed from two omni-directional acoustic sensors. The microphone includes a sound source acoustically coupled to the acoustic sensors and a processor. The sound source is excited with a test signal, exposing the acoustic sensors to acoustic calibration signals, which are of the same phase. The responses of the acoustic sensors to the calibration signals are compared by the processor and a correction transfer function is determined. The system is designed in particular for a dipole microphone.
MEMS microphones have been recently developed and find more and more applications of use. So far, little research has been conducted on the possibilities of use of the MEMS microphones for outdoor monitoring systems. MEMS microphones have very small dimensions, which allows designing a multi-microphone system having a housing of standard dimensions used in acoustic fields (for example, a ½″ or 1″ diameter). However, the MEMS microphones have no equivalent capacitance that could be measured, as in the case of condenser microphones and they cannot be excited by an electrostatic actuator.
A European patent application EP3223541 discloses an outdoor multi-microphone system with an integrated remote acoustic calibration system. The calibration system comprises a reference microphone for measuring ambient sound and a plurality of measurement microphones. In case the levels measured by the reference microphone and the measurement microphones differ by more than a particular threshold, a negative system check result is output.
The prior art solutions for checking microphone systems required interruption of the regular operation of the system in order to perform the system check and use of a dedicated reference microphone in order to perform the system check.